Direct imaging revealing halved ferromagnetism in tensile-strained LaCoO3 thin films

Abstract

The enigma of the emergent ferromagnetic state in tensile-strained LaCoO3 thin films remains to be explored because of the lack of a well agreed explanation. The direct magnetic imaging technique using a low-temperature magnetic force microscope (MFM) is critical to reveal new aspects of the ferromagnetism by investigating the lateral magnetic phase distribution. Here we show the experimental demonstration of the rare halved occupation of the ferromagnetic state in tensilestrained LaCoO3 thin films on SrTiO3 substrates using the MFM. The films have uniformly strained lattice structure and minimal oxygen vacancies (less than 2%) beyond the measurement limit. It is found that percolated ferromagnetic regions with typical sizes between 100 nm and 200 nm occupy about 50% of the entire film, even down to the lowest achievable temperature of 4.5 K and up to the largest magnetic field of 13.4 T. Preformed ferromagnetic droplets were still observed when the temperature is 20 K above the Curie temperature indicating the existence of possible Griffiths phase. Our study demonstrated a sub-micron level phase separation in high quality LaCoO3 thin films, which has substantial implications in revealing the intrinsic nature of the emergent ferromagnetism.

Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)

“…In contrast, the impact of CD on the O-Co-O distances (∼ 0.1 Å) is at least one order of magnitude larger. Therefore, we suggest CD to be the reason for the experimentally observed distinct O-Co-O distances (3.86 Å at 90 K splitting up symmetrically into 3.76/3.98 Å at 295 K, 6 with some domains remaining NM, as it has been observed very recently for LCO films 25 ), instead of e g orbital order assisted by Jahn-Teller distortions. Shimizu et al reported a symmetry preservation across the spin transition, which is incompatible with Jahn-Teller distortions.…”

“…[1][2][3] This holds in particular if the emerging FM domains are embedded in an insulating NM matrix. 25 Finally, we note that a similar electronic structure is obtained for all systems in the IS state throughout this work.…”

“…Experimental growth of LCO films on STO(001) is usually carried out at around 650-750 • C and 1-320 mTorr oxygen pressure, as summarized in Table II. Fuchs et al, 22 Freeland et al, 21 and Qiao et al 24 exposed the samples to a 200-750 Torr oxygen atmosphere after deposition; nevertheless, they observed a similar Curie temperature as Mehta et al, 20,23 Choi et al, 17 Biškup et al, 19 and Feng et al 25 The relatively high temperatures employed during the growth process legitimate our thermodynamic approach, which is strictly valid only in equilibrium. 42,68 The phase diagram in Fig.…”

“…This may offer an explanation why the 3×1 reconstruction of LCO films strained on STO(001) is sometimes not observed. 24,25 Both phases exhibit a low conductivity, in particular the semiconducting IS/IS/IS phase, but also the LS/IS/IS phase due to the insulating LS planes. Moreover, the coexistence of FM and insulating NM domains in LCO films may add to the resistivity of the samples.…”

“…[17][18][19][20] These stripes were shown to be related to a short/short/long modulation of the La-La distances along the [100] direction. 19 Moreover, an insulating 21 and ferromagnetic (FM) ground state emerges, with a Curie temperature of T C ≈ 80 K. 17,[19][20][21][22][23][24][25] Two distinct models have been put forward to explain these observations. The first model is based on an ordered arrangement of oxygen vacancies in every third Co (100) plane, 19 formally LaCoO 2.…”